As is well known, prospecting for minerals of commercial or other value (including but not limited to hydrocarbons in liquid or gaseous form; water e.g. in aquifers; and various solids used e.g. as fuels, ores or in manufacturing) is economically an extremely important activity. Those wishing to extract such minerals from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the minerals in a geological formation and also any difficulties that may arise in the extraction of the minerals to surface locations at which they may be used.
For this reason, over many decades techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before mineral extraction activities commence and also, increasingly frequently, while they are taking place.
Broadly stated, logging involves inserting a logging tool including a section sometimes called a “sonde” into a borehole or other feature penetrating a formation under investigation; and using the sonde to energize the material of the rock, etc., surrounding the borehole in some way. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Such passage of the energy alters its character. Knowledge of the attributes of the emitted energy and that detected after passage through the rock may reveal considerable information about the chemistry, concentration, quantity and a host of other characteristics of minerals in the vicinity of the borehole, as well as geological aspects that influence the ease with which the target mineral material may be extracted to a surface location.
Logging techniques are employed throughout mining industries, and also in particular in the oil and gas industries.
The borehole usually is several thousands of feet in length yet is narrow (being perhaps as narrow as 3 inches), although in practice such a borehole is almost never of uniform diameter along its length.
It is known to convey a logging tool to a downhole location inside drill pipe. As is well known, drill pipe is strong, hollow steel pipe formed in lengths (sometimes called “singles”) that are threaded at each end so that they can be joined to one another by screwing them together. Lengths of drill pipe that are thus connected together at a surface location are deployed sequentially into a borehole e.g. as drilling of the borehole takes place. This activity is sometimes referred to as “running in”.
Typically, a drill head including plural rotary drill bits is secured on the free end of the most downhole length of drill pipe and may be caused to rotate and in some cases is additionally powered by fluid pumped in a downhole direction via the interior of the drill pipe in order to cut the material ahead of the downhole end of the drill pipe. Such fluid is circulated within the borehole and, after passing through the drill head, flows externally of the drill pipe to return to a surface location for filtering and re-use, carrying the debris of the drilling operation with it out of the borehole.
It is known to cause rotation of the drill pipe, and hence of an attached drill bit, through operation of the rotary table or top drive (both of which terms are known to a person of skill in the art) of a derrick that supports and guides the drill pipe at the surface location.
It is also known to cause longitudinal reciprocation of the drill pipe. Typically this is achieved through the use of a form of top drive or other drill string drive/support that includes an elevator. The elevator in a typical installation is capable of powering the drill pipe to move up and down in the borehole by a distance of several meters.
Appropriate control and/or programmed elements can be employed to cause rotation and/or reciprocation (when the aforthaid kind of top drive is fitted) of drill pipe in a borehole. At their simplest such elements include potentiometers or similar devices that can be used to control the activation of the rotary drive, top drive or elevator. More sophisticated control elements, including designs that are programmable, are also possible.
Several techniques exist for conveying logging tools of various types (such as acoustic, nuclear, gamma and resistivity tools as are known in the relevant art) for use at the downhole end of a length of drill pipe inserted into a borehole. Such techniques include shuttling the logging tool (i.e. keeping it inside the drill pipe within a protective shuttle, from which the logging tool is caused to protrude once it is in the correct location); so-called “garaging”, as defined in patent no GB 2372057 A, in which the lowermost length of drill pipe acts in a similar manner to a shuttle and from which the logging tool is caused to protrude once the drill pipe has reached a desired depth in the borehole; and wireline drop-off deployment.
In the last-mentioned conveyance technique, a logging tool having a memory function is conveyed down-hole by wireline through the drill pipe and once deployed, projects into the openhole supported on a no-go at the bottom of the drill pipe.
When the drill pipe has reached total depth (the planned end of the well measured by the length of pipe required to reach the bottom) and then drawn back to allow a deployment space, a wireline drop off tool including one or more sondes is lowered into the drill pipe.
The sonde section(s) is/are detachable from the remainder of the drop-off tool, under certain controlled conditions. Typically there is a landing ring in the internal wall of the drill pipe, located near the mouth of the final (i.e. most downhole) piece of drill pipe, which receives a landing collar located on and protruding outwardly from the tool. The engagement (“landing”) of the landing ring and collar secures the tool and pipe rigidly one to another. When this engagement has occurred, the sonde part and the remainder of the drop-off tool are detached from one another and the drop-off tool is removed from the well via the wireline. This leaves the sonde section(s) in place to carry out logging activities.
The result of this sequence is that part of the logging tool protrudes beyond the end of the drill pipe and therefore is exposed in a way that permits logging of the formation. A further part of the logging tool remains inside the drill pipe and defines the described landing collar connection to the drill pipe. Logging may then take place while pulling drill pipe from the well (i.e., removal of the drill pipe out of the borehole), with the memory of the logging tool recording log data without any need for wireline communication with a surface location. Following completion of the logging activity the logging tool is retrieved from the drill pipe and the recorded data downloaded for processing and analysis.
A characteristic of all the conveyance techniques outlined above is that before logging commences the logging tool must become landed on the end of the drill pipe, partially protruding as described in relation to the wireline drop-off deployment technique, such that the drill pipe rigidly supports the lugging tool.
To this end, it is desirable, and often essential, for communication between the surface location (where control, processing and command apparatuses are located) and the downhole logging tool to occur. Permanently in the case of the shuttling and garaging deployment methods, and once the wireline has been withdrawn in the wireline deployment method, there exists no direct connection (aside from the drill pipe itself) between the surface location and the logging tool.
As mentioned, during use a fluid that typically is a drilling mud or a combination of drilling mud and other fluids is often circulated through the drill pipe and the annulus surrounding it. It is known in the art to effect hydraulic communication between the surface and a logging tool using coded pulses generated in the drilling mud. Such pulses when used for downlink communication propagate in a downhole direction inside the drill pipe and are detected by pressure-sensing parts of the logging tool and/or the drill head. These latter then cause e.g. movement of the logging tool from a retracted position inside drill pipe to a protruding position, the extension of landing components that secure the logging tool to drill pipe, or the extension of parts that cause withdrawal of the drill head to a position permitting passage of part of the logging tool. The received pressure pulses also can initiate electronic commands e.g. causing the logging tool to become active and commence logging of the borehole.
The logging tool can also send a pressure pulse uplink signal, for instance through the closing of an otherwise open fluid flow path on correct landing of the tool on the drill pipe.
Pressure pulse signaling however is associated with numerous disadvantages. One of these concerns the great uncertainty of pressure pulse propagation time in the drill pipe fluid. The time taken for the pulse to reach the logging tool can be highly variable depending on a number of factors that are known to the person of skill in the art. This in turn can cause logging engineers to wait a considerable time for an uplink signal to arrive back at the surface location, before concluding e.g. that the downlink communication was unsuccessful and requires repeating. Since rig time is very expensive, time wasted in this fashion is strongly undesirable.
Moreover, at least the downlink signaling is hard to achieve reliably. This often results from the fact that the pressure pulse signal is generated by modulating the operational speed of the pump installed at the surface location for the purpose of circulating the fluid in the borehole. There may be significant latency and imprecision in the response of the pump to speed change commands; and more prosaically a drilling engineer responsible for controlling the pump may not correctly interpret the requests of a logging engineer as to the pressure pulse waveform, amplitude or frequency required.